Technical Field
The present invention relates to a power conversion apparatus having a function of protecting power semiconductor devices such as semiconductor switching devices that form the power conversion apparatus from an overheating accident.
Background Art
Patent Literature 1 (Japanese Patent No. 3075303, in, e.g., paragraph [0006], FIG. 2, and the like) and Patent Literature 2 (Japanese Patent Application Publication No. 2009-17707, in, e.g., paragraphs [0048] to [0052], FIG. 1, FIG. 11, and the like) disclose related arts of a power conversion apparatus for variable velocity driving of electric motors in which a power semiconductor device such as a semiconductor switching device is mounted. The power conversion apparatus limits current flowing into a semiconductor device before the temperature of the semiconductor device exceeds an allowable temperature to destroy the semiconductor device, to thereby protect the semiconductor device and the power conversion apparatus from an overheating accident.
FIG. 9 is a block diagram associated with overheat protection of a semiconductor device disclosed in Patent Literature 1, and FIG. 10 is a diagram for describing the operation thereof.
In FIG. 9, reference numeral 100 designates a rectifying circuit connected to a three-phase AC power supply, 200 designates an inverter having semiconductor switching devices 201 to 206, and 300 designates an electric motor driven by the inverter 200. Reference numeral 400 designates a control device, 401 designates a current detector, 402 designates a PWM (pulse-width modulation) control unit, and 403 designates a thermistor. Reference numeral 404 designates a temperature detecting unit, 405 designates a junction temperature estimating unit, 406 and 407 are subtractors that calculate differences between preset temperatures 1 and 2 and a junction temperature Tj, respectively. Reference numeral 408 designates an operational amplifier, 409 designates a current limiting function, and 410 designates a comparator. Reference numeral 411 designates a current blocking function, and 412 designates a base driver that drives switching devices 201 to 206.
In this related art, the junction temperature estimating unit 405 estimates the temperature (the junction temperature) of a switching device and limits or blocks an output current by such a current limiting rate as illustrated in FIG. 10 when the estimated temperature exceeds preset temperatures 1 and 2 (T0, T1) to thereby protect the switching device from overheating. In this related art, since an algorithm for estimating the junction temperature is not directly related to the present invention, description thereof will not be provided.
FIG. 11 is a block diagram associated with overheat protection (a block diagram of a motor control device) of a semiconductor device disclosed in Patent Literature 2, and FIG. 12 is a flowchart illustrating the operation thereof.
In FIG. 11, reference numeral 501 designates a torque limit value calculating unit that calculates a torque limit value based on a target torque and the outputs of a rotation number calculating unit 507 and a highest temperature extracting unit 510. Reference numeral 502 designates a torque-current converting unit that calculates a d-axis current command value idr and a q-axis current command value iqr from the torque limit value. Reference numeral 503 designates a current control unit that calculates a d-axis voltage command value vdr and a q-axis voltage command value vqr from deviations between the d-axis current command value idr and the q-axis current command value iqr and a d-axis current id and a q-axis current iq output from a coordinate transforming unit 508, respectively. Reference numeral 504 designates an inverse coordinate transforming unit that transforms the d-axis voltage command value vdr and the q-axis voltage command value vqr to three-phase voltage command values. Reference numeral 505 designates an inverter control unit that generates a drive signal (gate signal) to be supplied to each of switching devices of the inverter 506 based on the three-phase voltage command values. Reference numeral 301 designates a three-phase motor driven by the inverter 506. Reference numeral 507 designates a rotation number calculating unit that calculates the number of rotations of the motor 301. Reference numeral 508 is a coordinate transforming unit that detects current components of respective phase coils of the motor 301 and transforms the current components to the d-axis current id and the q-axis current iq, respectively. Reference numeral 509 designates a device temperature estimating unit that estimates the temperature of each of the switching devices of the respective phases of the inverter 506 from the current components of the respective phase coils of the motor 301 and the measured temperature value of the inverter 506 before rotation of the motor. Reference numeral 510 designates a highest temperature extracting unit that extracts a highest temperature from the input estimated temperature values.
In this related art, in the processes of steps S11 to S16 in FIG. 12, when a highest temperature is extracted from the estimated temperatures of the switching devices of the respective phases of the inverter 506, and the highest temperature is smaller than a predetermined temperature threshold, the current is controlled in the processes of steps S18 to S21 so that the motor 301 is driven by the inverter 506.
Moreover, when the highest temperature exceeds the temperature threshold (step S17: NO), the torque of the motor 301 is corrected so as to decrease the torque (S22) to thereby decrease the generation loss of the switching devices to realize overheat protection.
In the torque correction step (S22), a torque limiting amount is determined in advance according to a difference between an estimated temperature value and a temperature threshold, for example, and a torque command value is decreased by a ratio proportional to the difference between the estimated temperature value and the temperature threshold.
However, in Patent Literature 1 described above, as obvious from FIG. 9, a current that flows actually is limited by limiting the current command of an electric motor, and semiconductor devices are protected from overheating. However, in a control system for controlling an electric motor, if the current command is limited directly, this may interfere with control of the electric motor. Thus, the control of the electric motor may become unstable and it may become difficult to realize both overheat protection and stable control of the electric motor during the overheat protection. This will be described briefly below.
FIG. 13 is a control block diagram of a permanent magnet synchronous electric motor disclosed in Patent Literature 3 (Japanese Patent Application Publication No. 2009-290929, in, e.g., paragraphs [0013] to [0026], FIG. 1, FIG. 2, and the like) and FIG. 14 is a block diagram illustrating a configuration of a current command calculating unit 603 in FIG. 13. According to this related art, it is possible to utilize a reactance torque of a permanent magnet-type synchronous electric motor such as an embedded magnet-type synchronous electric motor and to generate a desired torque stably with a minimum necessary (that is, smallest) current.
Hereinafter, Patent Literature 3 will be described briefly with reference to FIGS. 13 and 14, and then the problem of Patent Literature 1 will be described.
First, the control block diagram of FIG. 13 illustrates the functions for controlling the velocity of an electric motor. A subtractor 601 calculates a deviation between a velocity command ω* and a detected velocity value ω1 of an electric motor 302, and a velocity regulator 602 adjusts a torque command τ* according to the deviation so as to obtain a desired rotating velocity.
Subsequently, the current command calculating unit 603 calculates d- and q-axis current commands id* and iq* obtained by rotationally transforming the coordinates of the current flowing in the electric motor 302. Here, as described above, in order to output a largest torque with a smallest current, the current command calculating unit 603 calculates optimal d- and q-axis current commands id* and iq* by taking the detected velocity value ω1 and the output (voltage limit value) valim of a voltage limit value calculator 612 based on a detected DC voltage value Edc output from a voltage detecting unit 611 into consideration.
Under d- and q-axis voltage commands id* and iq*, subtractors 604d and 604q and d- and q-axis current regulators 605d and 605q calculate d- and q-axis voltage commands vd* and vq* so that the values id and iq obtained by a current coordinate transformer 614 rotationally transforming the coordinates of detected current values iu and iw (and iv) detected by current detectors 613u and 613w become the d- and q-axis current command values id* and iq*.
A voltage coordinate transformer 606 transforms the d- and q-axis voltage commands vd* and vq* to U-, V-, and W-phase voltage commands vu*, vv*, and vw* and transmits the voltage commands to a PWM circuit 607. The PWM circuit 607 performs PWM control while taking the DC voltage Edc into consideration to generate gate signals of semiconductor switching devices that form a power converter 610 such as an inverter.
Reference numeral 608 designates a three-phase AC power supply, 609 designates a rectifying circuit, 615 designates a pole position detector, and 616 designates a velocity detector.
Moreover, as illustrated in FIG. 14, in the current command calculating unit 603, a magnetic flux command value ψ* and a load angle command value δ* are calculated by the operations of a magnetic flux command calculator 603a, a load angle command calculator 603b, a load angle regulator 603d, a magnetic flux limit value calculator 603e, an output limiter 603f, a torque calculator 603j, a subtractor 603c, an adder 603g, and the like. Moreover, the d- and q-axis current commands id* and iq* are calculated by a current command calculator 603h. 
The torque calculator 603j calculates an output torque τcalc of the electric motor based on the d- and q-axis current commands id* and iq* calculated by the current command calculator 603h, and the calculated torque value τcalc is fed back so that the load angle δ* is adjusted so as to match a torque command τ*. In particular, when the voltage necessary for the power converter 610 in FIG. 13 to drive the electric motor 302 is not sufficient, the load angle regulator 603d operates to limit the magnetic flux inside the electric motor 302 based on a calculation result obtained by a magnetic flux limit value calculator 603e. 
By using such a control method, it is possible to utilize the reactance torque of a permanent magnet-type synchronous electric motor such as an embedded magnet-type synchronous electric motor and to control the velocity of the electric motor with a desired torque and a smallest current stably.
FIG. 13 is a control block diagram for controlling the velocity of the electric motor 302 as described above. Depending on an apparatus to which the electric motor is applied, simple torque control may be performed. In this case, the torque command τ* is input directly from the outside instead of using the velocity regulator 602 in FIG. 13.
As described above, in Patent Literature 3, optimal d- and q-axis current commands id* and iq* are calculated based on the torque command τ* of the electric motor 302, the output of the power converter 610, and the like.
However, if a current limiting unit for protecting a semiconductor device from overheating is provided at the subsequent stage of the current command calculator 603h in FIG. 14, for example, using the technique disclosed in Patent Literature 1 so as to just limit the magnitude of the output current value (to limit the magnitude of any one or both of the d-axis current command id* and the q-axis current command iq*), the control method described in FIG. 14 and the overheat protection method may interfere and it may be difficult to control the electric motor 302 stably.
In order to obviate this problem, although the control method described in FIG. 14 may be modified for improvement, the improved control method may become complex and the control device may become expensive.
Next, the problem of the related art disclosed in Patent Literature 2 will be described.
According to Patent Literature 2, overheating of a switching device can be prevented by decreasing the torque command value. Thus, when this technique is applied to the related art of Patent Literature 3, the value τ* described in FIG. 14 may be decreased.
However, the torque decrease amount calculating unit disclosed in Patent Literature 2 has the following problem.
The temperature of a semiconductor device will be described before describing the problem of Patent Literature 2 is described in detail.
FIGS. 15 and 16 are examples of simulation results of a temperature rise value and the like of a semiconductor device in relation to a coolant. A power semiconductor module used in this simulation is a direct liquid cooling-type power semiconductor module disclosed in Non-Patent Literature 1 (“Direct Liquid Cooling IGBT Module for Automotive Applications,” Fuji Electric Review, Vol. 84, No. 5, pages 308-312, 2011) described later. Here, a direct liquid cooling system is a system in which heat generated by the power semiconductor module is dissipated directly to cooling water as a coolant, and the details thereof are disclosed in Non-Patent Literature 1. The absolute temperature of the semiconductor device in such a power semiconductor module is an addition of the temperature rise value described in FIGS. 15 and 16 and a coolant temperature.
FIGS. 15 and 16 illustrate simulation results of an electric motor current iu [A] when a certain electric motor as a control target of a power conversion apparatus outputs a certain torque trq [N·m], a generation loss T_UP_LOSS [W] of a certain semiconductor device among a plurality of semiconductor devices that form the power conversion apparatus, and a temperature rise value T_UP_TJW [K] of the semiconductor device in relation to the coolant. The difference between both figures is an output frequency as an operation condition. As obvious from FIGS. 15 and 16, since the same torque is generated even if the output frequency is different, the amplitude of a current flowing in the electric motor and the generation loss are the same under both conditions.
However, FIG. 15 in which the output frequency is lower than the other figure shows a higher temperature rise value T_UP_TJW of the semiconductor device in relation to the coolant. This is because, although the average temperature of both cases is the same since a relaxation time is present between the generation loss of the semiconductor device and the temperature rise associated therewith, and because the lower the output frequency, the higher the spontaneous temperature becomes.